The invention relates to a means for releasing the locking mechanism of a prosthetic elbow. Various artificial elbows have been suggested which include a mechanism for locking the elbow in a chosen, flexed position and for releasing the locked elbow when further movement is desired. In U.S. Pat. No. 2,812,961 to Brown et al, a prosthetic locking hinge for an artificial arm is described wherein a spring biases a sliding tooth into engagement with a gear sector such that the humeral section of the arm is maintained in an angular relationship with respect to the forearm section via the now rigid hinge. Shoulder movement by the prosthesis wearer tenses a cable which disengages the tooth from the gear sector, enabling the elbow to bend to a new position. A similar mechanically operated device is disclosed in U.S. Pat. No. 3,382,506 to Collins et al, wherein a single pull-cord is utilized for unlocking the elbow joint to permit angular motion of the forearm. The elbow joint includes an arcuate toothed member which is engaged by a latch mounted in the upper arm. The pull-cord is connected via pulleys to a sliding device for engaging (or withdrawing) the latch with (or from) the toothed member. As the latch disengages, the toothed member rotates. In U.S. Pat. No. 4,038,706 to Ober et al, an elbow mechanism is shown having a control cable which rides on a first pulley in the forearm and a second pulley accommodated inside a spring-biased sliding lock, also in the forearm. An initial tension on the cable by the wearer withdraws the lock from a cut-out in the arm against the force of the biasing spring, while further tension on the cable causes the elbow to bend.
Protheses have also been proposed which include a drive motor for powered flexing or movement thereof. U.S. Pat. No. 3,735,425 to Hoshall et al discloses a myoelectrically controlled prosthesis with a closed-loop servo system comprising a sensor, amplifier, control unit and power pack. An artificial hand opens in direct proportion to control signal amplitude as the muscles of the wearer are contracted. Hoshall et al do not discuss a flexing elbow, but only address operation of a prosthetic hand via the motor. Seamone et al, in U.S. Pat. No. 4,067,070, describe a motor-driven prosthetic arm having an elbow joint lock and cable mechanism. The mechanism includes an electronic circuit which couples a solenoid to a motor. The solenoid is activated to control a radially disposed pawl member which slidably engages and disengages a toothed wheel to respectively lock and unlock the elbow. A first pulse is supplied to a lock-solenoid, whereby a ratchet wheel is locked by a pawl to prevent opening of a grasping hook assembly. Immediately following the first pull, the ratchet wheel is released and a second pulse is generated which locks the elbow, continued energization of the motor developes cable tension sufficient to open the grasping hook assembly. The elbow is unlocked when a releasing cable is tensed. A substantially similar device is disclosed in Loveless' U.S. Pat. No. 4,074,367 for a prosthetic load-lift hook locking mechanism. The pawl engages teeth of the ratchet wheel as long as there is steady tension on the cable which urges the ratchet wheel (via an integral pulley) counter clockwise. However, Loveless teaches that once the elbow is locked and the cable tension is released, a spring allows clockwise rotation of the ratchet wheel, permitting the pawl to clear the ratchet wheel teeth.
When an object is lifted with a prosthesis, the object's weight creates a downward force at the prosthesis end, generating a substantial torque at the elbow joint. If the elbow is locked in the manner of the prior art devices a locking elbow pawl is engaged with a tooth of an elbow gear. Owing to the torque generated by the lifted weight, the elbow gear tends to turn against the locking pawl such that a great frictional force is present between the locking pawl and the adjacent elbow gear tooth. When the amputee attempts to unlock the elbow joint by tensing a release cable connected to the locking pawl, the frictional force is of such magnitude that the locking pawl cannot be dislodged from the elbow gear and further bending of the elbow is prohibited.